1. Technical Field
The present invention relates to liquid crystal display panels adopting the horizontal electric field mode and, more particularly, to a liquid crystal display panel adopting the horizontal electric field mode, which includes an upper electrode having slit-shaped apertures and operating as a common electrode.
2. Field of Invention
Liquid crystal display panels, which are characterized by lightness in weight, small thickness and low power consumption as compared with cathode-ray tubes (CRTs), have been used for various types of electrical devices as displays incorporated therein. A principle of displaying images employed in such a liquid crystal display panel is such that, an amount of transmitted light or an amount of reflected light varies in accordance with alignment direction changes of individual liquid crystal molecules in proportion to intensity of an electric field applied to the liquid crystal molecules, each of which is initially aligned in a prescribed direction resulting from a rubbing process performed on alignment films having the liquid crystal molecules interposed therebetween.
There are two schemes in methods of applying an electric field to a liquid crystal layer included in a liquid crystal display, one being a vertical electric field mode, the other one being a horizontal electric field mode. In a liquid crystal display panel adopting the vertical electric field mode, an electric field, which is generated by a pair of electrodes having a liquid crystal layer interposed therebetween and extends in a direction substantially vertical to the pair of electrodes, is applied to liquid crystal molecules included in the liquid crystal layer. For liquid crystal displays adopting the vertical electric field mode, various modes, such as a twisted nematic (TN) mode, a vertical alignment (VA) mode and a multi-domain vertical alignment (MVA), are well known to those skilled in the art. In a liquid crystal display panel adopting the horizontal electric field mode, an electric field, which is generated by a pair of electrodes formed in a mutually insulated condition at the inside side of one of the pair of substrates having a liquid crystal layer interposed therebetween and extends in a direction substantially horizontal along the pair of electrodes, is applied to liquid crystal molecules included in the liquid crystal layer. For the liquid crystal display panels adopting the horizontal electric field mode, an in-plane switching (IPS) mode, in which the pair of electrodes are formed so as not to be overlapped in plan view, and a fringe field switching (FFS) mode, in which the pair of electrodes are formed so as to be overlapped in plan view, are well known to those skilled in the art.
Among these liquid crystal display panels employing various modes, a liquid crystal display panel employing the FFS mode includes a plurality of pairs of a common electrode and a pixel electrode, the common electrode and the pixel electrode being located on different layers, respectively, and having an insulating film interposed therebetween, and further, provides the common electrode or the pixel electrode located at a liquid crystal layer side with slit-shaped apertures through which an electric field extending in a substantially horizontal direction is applied to the liquid crystal layer. Utilization of such a liquid crystal display panel employing the FFS mode leads to certain advantages, that is, a large viewing angle and improvement of image contrast, and therefore, nowadays, the liquid crystal display panels employing the FFS mode have been in widespread use. For liquid crystal display panels employing the FFS mode, there are two types which are well known to those skilled in the art, one being a type in which pixel electrodes are formed on planes the same as those on which thin-film-transistors (TFTs) operating as switching elements are formed (refer to JP-A-2007-327997), the other one being a type in which both the pixel electrodes and the common electrodes are located above TFTs (refer to JP-A-2008-32899).
Out of these types, in a liquid crystal display panel of the type in which both the pixel electrodes and the common electrodes are located above TFTs, the surfaces of TFTs and the like are coated by a interlayer film composed of a resin layer, and on the surface of this interlayer film, lower electrodes composed of a transparent conductive material are formed. Further, the lower electrodes can be operated as either the pixel electrodes or the common electrodes. In the case where the lower electrodes are allowed to operate as the pixel electrodes, contact holes are formed in the interlayer film, and through the contact holes, the pixel electrodes are electrically connected to the corresponding switching elements. Moreover, on the surface of an insulating film formed so as to cover the lower electrodes, an upper electrode composed of a transparent conductive material is continuously formed across a plurality of pixel areas. This upper electrode has sections partitioned in accordance with the pixel areas, and in each of the sections, a plurality of slit-shaped apertures are formed. The upper electrode is electrically connected to common wiring formed in an area surrounding a display region and operates as the common electrode. Out of the liquid crystal display panel types in which pairs of an upper electrode and a lower electrode, which are both located above TFTs, are formed, the liquid crystal display panel employing the FFS mode including the lower electrodes and the upper electrode operating as the pixel electrodes and the common electrode, respectively, has advantages that, respective areas of apertures located near TFTs and boundaries between pixel areas become larger, and no wiring associated with the common electrode exists within the display region, and as a result, a higher aperture ratio can be achieved.
However, the above-described liquid crystal panel employing the FFS mode has a disadvantage that, allowing the common electrode including sections each having slot-shaped apertures to be formed continuously across a plurality of pixel areas causes the common electrode to be electrically connected to the exterior in peripheral areas surrounding the display region, and as a result, the electrical resistance of the common electrode increases, and this increase of the electrical resistance of the common electrode results in occurrence of defects such as flickering and electrical crosstalk. On the other hand, for the liquid crystal display panels employing the IPS mode, as disclosed in, for example, JP-A-2005-258408, a method in which, in order to prevent increasing of respective electrical resistances of the pixel electrodes and the common electrode resulting from forming both the pixel electrodes and the common electrode by using a transparent conductive material, both the pixel electrodes and the common electrode are formed so as to have dual structures which are composed of a transparent electrode and a metallic electrode, is well known to those skilled in the art. However, in the case of the liquid crystal display employing the FFS mode, the pixel electrodes and the common electrode are overlapped in plan view, and as a result, the method of applying the dual structure composed of a transparent electrode and a metallic electrode to both the pixel electrodes and the common electrode leads to lowering of the aperture ratio, and thus, it is not preferable to adopt this method.